Versioned cargo wrapper script points to default cargo

[Simon Chopin]

[ Impact ]

 * If you use the Cargo wrapper script provided by a versioned
   cargo package, e.g. /usr/lib/rust-1.85/share/cargo/bin/cargo,
   it will invoke /usr/bin/cargo rather than
   /usr/lib/rust-1.85/bin/cargo.

 * This means that users expecting a particular Cargo version are
   actually invoking the rust-defaults Cargo version behind the
   scenes.

 * Additionally, if the rust-defaults Cargo version isn't
   installed, the versioned Cargo wrapper script doesn't work at
   all (because it's trying to invoke the nonexistent default).

 * As stated in the comments of this bug report, this is blocking
   the update of authd's Rust dependencies.

 * This upload fixes the bug by making the Cargo wrapper script
   invoke the versioned Cargo instead.

 * In addition, it adds a new "versioned-cargo-wrapper" autopkgtest
   which verifies that the Cargo version invoked by the wrapper
   script matches the version of the package itself.

[ Test Plan ]

 * To reproduce on Noble, do the following:
   - # apt install -y cargo-1.85
   - $ /usr/lib/rust-1.85/share/cargo/bin/cargo --version
     - This should produce a FileNotFoundError.
   - # apt install -y cargo
   - $ /usr/lib/rust-1.85/share/cargo/bin/cargo --version
     - This should (incorrectly) show cargo 1.75.0.

 * To test the fix on Noble, do the following:
   - # add-apt-repository -y ppa:maxgmr/rustc-lp2100266
   - # apt update && apt upgrade -y
   - # apt remove -y cargo
   - $ /usr/lib/rust-1.85/share/cargo/bin/cargo --version
     - This should now show cargo 1.85.1 (instead of FileNotFound)
   - # apt install -y cargo
   - $ /usr/lib/rust-1.85/share/cargo/bin/cargo --version
     - This should also show cargo 1.85.1 (instead of 1.75.0)

 * Additionally, the new "versioned-cargo-wrapper" autopkgtest
   should pass.

[ Where problems could occur ]

 * This upload changes the toolchain version which is invoked by the
   versioned Cargo wrapper. As a result, applying this SRU means that
   packages will be built with a different Rust version.

 * This means that, for example, on Noble, packages built with
   cargo-1.85 were being built with 1.75 the whole time. When the
   SRU is applied, all these packages will be (correctly) built with
   1.85 as intended, but this could potentially cause builds to fail.

 * This change does not affect the vast majority of Rust packages, as
   they are all built with the default Rust version.

[ Other Info ]

 * In d/rules, my exclusion of 'librustc_driver' from the dh_dwz step
   is not related to this bug, but the build failed otherwise, so I
   needed to include this fix as well.

[Ural Tunaboyu (uralt)]

Ubuntu 24.10 (Oracular Oriole) has reached end of life, so this bug will not be fixed for that specific release.

[Max Gilmour (maxgmr)]

I've created fixes for several of the targeted versions. Their debdiffs shall be included in future comments.

======
Before
======

root@plucky-bug:~# apt install -yU cargo cargo-1.82
[...]
root@plucky-bug:~# cargo --version
cargo 1.84.1 (66221abde 2024-11-19)
root@plucky-bug:~# /usr/lib/rust-1.82/share/cargo/bin/cargo --version
cargo 1.84.1 (66221abde 2024-11-19)

=====
After
=====

root@plucky-bug:~# add-apt-repository ppa:maxgmr/rustc-lp-2100266
[...]
root@plucky-bug:~# apt-cache policy cargo-1.82
cargo-1.82:
  Installed: 1.82.0+dfsg0ubuntu1-0ubuntu2
  Candidate: 1.82.0+dfsg0ubuntu1-0ubuntu3~ppa2
  Version table:
     1.82.0+dfsg0ubuntu1-0ubuntu3~ppa2 500
        500 https://ppa.launchpadcontent.net/maxgmr/rustc-lp-2100266/ubuntu plucky/main amd64 Packages
 *** 1.82.0+dfsg0ubuntu1-0ubuntu2 500
        500 http://archive.ubuntu.com/ubuntu plucky/universe amd64 Packages
        100 /var/lib/dpkg/status
root@plucky-bug:~# apt install cargo-1.82
[...]
root@plucky-bug:~# cargo --version
cargo 1.84.1 (66221abde 2024-11-19)
root@plucky-bug:~# /usr/lib/rust-1.82/share/cargo/bin/cargo --version
cargo 1.82.0 (8f40fc59f 2024-08-21)

[Adrian Dombeck (adombeck)]

The issue doesn't seem to be fixed on noble, the cargo wrapper still uses /usr/bin/cargo:

$ sudo apt-cache policy cargo-1.85
cargo-1.85:
  Installed: 1.85.1+dfsg0ubuntu2~bpo0-0ubuntu0.24.04
  Candidate: 1.85.1+dfsg0ubuntu2~bpo0-0ubuntu0.24.04
  Version table:
 *** 1.85.1+dfsg0ubuntu2~bpo0-0ubuntu0.24.04 500
        500 http://archive.ubuntu.com/ubuntu noble-updates/universe amd64 Packages
        500 http://security.ubuntu.com/ubuntu noble-security/universe amd64 Packages
        100 /var/lib/dpkg/status

$ /usr/lib/rust-1.85/share/cargo/bin/cargo --version
cargo 1.75.0

Am I doing something wrong?

[Adrian Dombeck (adombeck)]

Oh, I think I misunderstood your last comment. I now see that the status is "In Progress", not "Fixed".

[Adrian Dombeck (adombeck)]

This is still blocking the update of authd's Rust dependencies. Can we get this fixed in rustc-1.85 in Noble?

[Max Gilmour (maxgmr)]

> Can we get this fixed in rustc-1.85 in Noble?

Hi Adrian,

Thank you for your patience on this issue. I've finished the Noble rustc-1.85 fix and I'll get it uploaded ASAP.